This invention relates to cardiac electrotherapy, and particularly to measurement of blood flow velocity characteristics within the heart and large blood vessels for the purpose of control of the electrotherapy.
Physiologic cardiac pacing is very important on a temporary as well as on a permanent basis. Temporary pacing is usually applied either after cardiac surgery or during myocardial infarction because of the transient conduction disturbance or arrhythmia. Patients at rest have significantly improved cardiac output when ventricular contraction is synchronous with atrial filling of ventricles. This is very important for faster recovery after surgery or myocardial infarction. Furthermore, some arrhythmias like supraventricular tachycardias and extrasystolies may be prevented by means of physiologic pacing.
Patients with chronic conduction and rhythm disturbance have to receive a permanent implantable pacing system. They also have a significant contribution of atria to the hemodynamic benefit. There are two basic modes of physiologic cardiac pacing: sequential and synchronous. The sequential atrio-ventricular pacing is used to restore normal atrio-ventricular relationships. In this mode an atrium and a ventricle are paced by twin stimuli separated by an appropriate physiologic interval. However, the heart rate is controlled by the pacemaker program and does not vary according to the physiological needs. The synchronous cardiac pacing approximates most closely to the normal cardiac rhythm. The spontaneous atrial electrogram (P-wave) is sensed by an electrode usually in contact with the atrial endocardium. This is used to trigger the ventricle after an appropriate preset delay. Because the atrial rhythm is paced by our natural pacemaker sinus-atrial node, the frequency varies naturally according to the body workload. Therefore, the P-wave synchronous ventricular cardiac pacing is considered to be the most physiologic rate-responsive pacing.
There is a significant drawback of physiologic pacing systems which complicates the surgical procedure in comparison with non-physiologic pacing. The physiologic pacing requires the implantation of two leads: one atrial and one ventricular. Modern dual-chamber pacemakers have the ability to switch from sequential to synchronous pacing and vice versa according to the atrial rhythm which is monitored in the atrial channel. If the patient has a normal function of the sinus node and atria, the atrial lead is only used to sense the atrial activity and the ventricular lead is used to sense the ventricular activity and to pace the ventricles. Because the sensing of atrial activity may be done by an electrode floating within the right atrial cavity, a lot of effort has been done to design a single pass lead for P-wave synchronous ventricular pacing comprising the atrial and ventricular electrode on the same lead. Such a system has been described in U.S. Pat. No. 3,903,897. However, the atrial electrogram has a significantly lower amplitude when sensed by a floating electrode in comparison with an electrode having a direct contact with the atrial muscle. Therefore, such systems have to have a high sensitivity amplifier in the atrial channel. As a consequence, the high susceptibility on far fields appears, causing more likely occurrence of the various oversensing phenomena. Furthermore, many patients have a low amplitude atrial electrogram and therefore the atrial undersensing is more frequent in such systems. The system described in the European Patent No. 311,019 monitors ventricular impedance continuously using an electrode in the ventricle without requiring additional sensing in the atrium. A detected impedance waveform can be used to trigger ventricular stimulus synchronously with atrial filling of the ventricle. In this system, the impedance changes because of the ventricular volume change caused by the atrial filling.
Very important to technical and clinical performance of P-wave synchronous pacemakers is the upper rate behavior. A maximum pacing rate of ventricles is limited and therefore the atrial rhythm tracking by the ventricles will happen within the specified frequency range. The maximum tracking rate has to be a programmable parameter in order to tailor the pacing frequency range according to the patient's needs. Those who suffer from angina pectoris and impaired ventricular function are not capable of tolerating high tracking rates, while those with a healthy cardiac muscle can tolerate high rate ventricular pacing. The synchronous pacing can be impaired by the atrial undulation and fibrillation when the pacemaker sustains the maximum tracking rate during high atrial pathologic rhythm. therefore, even the intermittent atrial fibrillation is a contraindication for synchronous pacing. Patients suffering from intermittent atrial fibrillation would benefit a lot from a pacemaker comprising a reliable atrial fibrillation detector and which could switch from synchronous to rate responsive pacing in the case of atrial fibrillation occurrence and vice versa, and switch back to the synchronous mode upon the fibrillation termination. It would be very important that a pacemaker could monitor the ventricular performance and adapt the maximum tracking rate in such a way as to prevent angina and high-rate induced ischemia. It would be also important that a pacemaker could discriminate premature ventricular contractions with compensatory pause from those without the compensatory pause. Tachycardia is a condition in which the heart beats rapidly. Pathologic tachycardia is the one which disturbs the hemodynamics causing the drop of systemic blood pressure. There are many types of pathologic tachycardias, and the electrophysiology differentiates two major classes: supraventricular and ventricular tachycardias. Tachycardia is often the result of electrical feedback within the heart structures where the natural beat results in the feedback of an electrical stimulus which prematurely triggers another beat. There are several different cardiac pacing modes which may terminate the tachycardia. The underlying principle in all of them is that if a pacemaker stimulates the heart at least once shortly after a heartbeat (and before the next naturally occurring heartbeat at the rapid rate), the interposed stimulated heartbeat disrupts the stability of the feedback loop, thus reverting the tachycardia to sinus rhythm. Such a pacemaker was disclosed in U.S. Pat. No. 3,942,534 which, following detection of tachycardia, generates a stimulus after a delay interval.
The most hazardous arrhythmia is ventricular tachycardia which may progress into lethal arrhythmia ventricular fibrillation. Because the ventricular tachycardia is not always successfully treated and terminated by antitachycardia pacing, the implantable cardioverter-defibrillator is used to deliver a high energy pulse shock in order to cause the cardioversion of ventricular tachycardia to sinus rhythm. Such an implantable device was disclosed in U.S. Pat. No. 4,614,192 comprising a bipolar electrode for R-wave sensing, the system utilizing heart rate averaging and a probability density function for fibrillation detection. The similar system for a cardioversion is disclosed in U.S. Pat. No. 4,768,512 which has the high frequency pulse delivery. All these systems deliver high energy shock through special patch-electrodes such as described in U.S. Pat. No. 4,291,707. In order to simplify the surgical procedure, systems comprising a superior vena cava electrode and a subcutaneous electrode, such as described in U.S. Pat. No. 4,662,377, have been developed. The supraventricular tachycardia caused by atrial flutter or fibrillation can be also treated by an implantable cardioverter, such as described in U.S. Pat. No. 4,572,191.
The difficulty in the electrotherapy treatment of tachycardia is that the implantable apparatus has to comprise means for the accurate detection of pathologic tachycardia in order to deliver the electrotherapy pulses whenever the pathologic tachycardia occurs. The problem is that the heart rhythm increases its frequency physiologically whenever either physical or emotional stress occurs. The means for pathologic tachycardia detection must accurately differentiate the natural sinus tachycardia which should not be treated by means of electrotherapy, from the pathologic tachycardia which has to be treated. Therefore, the discrimination between normal and pathologic tachycardia on the basis of frequency measurement is not reliable. In order to overcome this problem, numerous methods of tachycardia detection have been developed which are applicable in the implantable electrotherapy devices.
Such a system has been disclosed in U.S. Pat. No. 4,475,551 where the heart rate sensing as well as the probability density function are used to distinguish between ventricular fibrillation and high rate tachycardia. A more sophisticated system has been disclosed in U.S. Pat. No. 4,790,317 which can automatically recognize the pathologic rhythm by means of monitoring of the pulse sequence representing the ventricular electrical activity. At least two sensing positions, i.e. to each ventricular epicardial surface, are used, but more sensing points will obtain better discrimination between normal and pathologic rhythms.
The problems which may occur with such systems are that they are susceptible to electromagnetic interference and muscular noise, as well as improper gain of the heart beat detectors causing the undersensing of cardiac rhythm. Therefore, some means for detecting of noise and for automatic sensitivity adjustment is desirable. Therefore, the implanted pacemaker noise rejection system described in the U.S. Pat. No. 4,779,617, as well as the automatic sensitivity control systems disclosed in U.S. Pat. Nos. 4,766,902 and 4,768,511 have been developed.
The implantable cardioverting system usually comprises the cardiac pacing system because of the backup of bradycardial events which follow the cardioversion high voltage pulse. There are also patients who suffer from pathologic tachycardia as well as from bradycardia, which have to be treated by cardiac pacing. therefore, the physiological sensor for control of the heart rate is desirable in order to obtain the rate responsive pacing. It is also possible that the cardioversion implantable device comprises a dual chamber physiologic pacing function. In such a system, a sensor for atrial fibrillation detection would be important, not only for the appropriate ventricular response on atrial rhythms, but also for differentiating supraventricular from ventricular tachycardia.
There are many physiological control systems for rate responsive pacing, but only a few of them can be used for tachycardia detection as well. As far as it is known to the inventors, none of these sensor systems can be used for ventricular tachycardia detection, rate responsive pacing, for atrial fibrillation detection, for pacing capture, and for noise detection. The system disclosed in U.S. Pat. No. 4,774,950 comprises a circulatory systematic blood pressure measurement system which detects the drop of pressure in the case of pathologic heart rhythm. A similar system is described in U.S. Pat. No. 4,791,931 where the pressure is measured by means of arterial wall stretch detection. Another system disclosed in U.S. Pat. No. 4,770,177 adjusts the pacing rate relative to changes in venous blood vessel diameter that is measured by means of a piezoelectric sensor. The heart contractions change the ventricular chamber volume due to the inflow and outflow of blood, thus varying the impedance within the chamber. The impedance measurement was used in U.S. Pat. No. 4,773,401 in order to obtain the physiological control of pacing rate. Furthermore, the stroke volume and ventricular volume measurement is possible in the system described in U.S. Pat. No. 4,686,987 as well as in U.S. Pat. No. 4,535,774. The system disclosed in U.S. Pat. No. 4,802,481 comprises a transducer which detects the opening of the tricuspid valve in order to calculate the ejection time, which is the sensor for rate responsive pacing. Obviously, all these systems measure indirectly the mechanical contraction of the heart which is the consequence of the electrical depolarization and which has the performance influenced by a sympathetic and a parasympathetic nervous system as well as by circulatory catecholamines. The sympathetic stimulation and circulatory catecholamines increase the velocity of the contraction, and therefore the hemodynamic forces are accordingly transferred to the circulatory system. In the case of pathologic rhythm having an electric depolarization disturbance, hemodynamics will be impeded. The quality of the mechanical cardiac contraction significantly differs in normal and pathologic rhythms. Not only the contraction but also the cardiac relaxation is influenced by circulatory catecholamines. In pathologic cardiac rhythm, the relaxation of the heart will be critically impeded. As far as it is known to the inventors, none of the systems used the parameters of cardiac relaxation, i.e. diastole for the cardiac electrotherapy control.
Ultrasonic measurement of blood flow has recently become an important noninvasive diagnostic method. Two methods have emerged as practical, i.e. the continuous wave (CW) and the pulsed (PW) Doppler systems. Very sophisticated and clinically useful systems have been developed such as described in U.S. Pat. No. 4,790,322 enabling automatic measuring independent to direction of ultrasonic beam emission. The ultrasonic transmitter-receiver for blood velocity measurement was described in U.S. Pat. No. 4,766,905 having improved noise reduction. Another system disclosed in U.S. Pat. No. 4,771,789 calculates and displays acceleration of a moving reflective member in an organism. A flow imaging detector for blood velocity measurement, such as disclosed in U.S. Pat. No. 4,790,323, weights samples of an auto-correlation function with reliability criterion so electrical noise dominated samples can be weighted less. All these inventions enabled the perfect imaging of the blood flow in the echocardiographic scanner image. Nevertheless, in some clinical applications, more accuracy was necessary and therefore the ultrasonic invasive methods have been introduced. An apparatus with a catheter for ultrasonic examining of hollow organs was described in U.S. Pat. No. 3,938,502. With continuing miniaturization of the apparatus, the idea of measuring blood flow or other parameters with piezoelectric transducers mounted on catheters (cardiac or other) became feasible. The localization and visualization systems have been developed which enabled the ultrasonic guidance of invasive procedures. The ultrasonic needle tip localization system was disclosed in U.S. Pat. No. 4,249,539. The ultrasonically marked catheters and cardiac pacing leads have been described in U.S. Pat. Nos. 4,697,595 and 4,706,681, respectively.
A particular problem to be solved is the measurement of the blood flow characteristics within the heart and large blood vessels. The system disclosed in U.S. Pat. No. 4,319,580 was developed to detect air emboli in the blood by using a cylindrical transducer for the detection. This approach was adequate for strongly reflective objects such as emboli and for the specified task of essentially only detecting them. The approach, however, does not yield a possibility to measure the flow characteristics as needed for pacemaker control.
Along similar lines there have been developed devices for measurement and control of large vessel blood flow estimation and cardiac output measurement as per U.S. Pat. No. 4,771,788 and U.S. Pat. No. 4,802,490. Apart from its use as a Doppler transducer, the device described in the U.S. Pat. No. 4,802,490 is from the ultrasonics point of view equal to the devices described in U.S. Pat. Nos. 4,706,681 and 4,697,595, although it has an additional flow restriction device which is immaterial in the comparison of prior art for the present application. The device described in U.S. Pat. No. 4,771,788 has basically the same ability to measure the flow by means of ultrasound, but is not suitable for implantation in the human body as a part of an electrotherapy system. This is so because it requires an additional support wire, which for different purposes may be helpful, but rules the method out for the aforementioned purposes.
Physiologic cardiac pacing is very important on temporary as well as on a permanent basis. Temporary pacing is usually applied either after cardiac surgery or during myocardial infarction because of the transient conduction disturbance or arrhythmia. Patients in rest have significantly improved cardiac output when ventricular contraction is synchronous with atrial filling of ventricles. This is very important for faster recovery after surgery or myocardial infarction. Furthermore, some arrhythmias like supraventricular tachycardias and extrasystolies may be prevented by means of physiologic pacing.